Thanks to developments in manufacturing high-Tc superconductor coated-conductor superconducting tapes, high-Tc superconducting (HTS) coils are gradually being more commonly used for magnets for example in MRI (magnetic resonance imaging), motor windings, and so forth. HTS coils can be powered by an external power source, or may operate in a persistent current mode. High-Tc Superconducter (HTS) Coated Conductors (CCs) are ideal candidates for high field magnets use, because of the high critical field of high-Tc superconductors as well as the good mechanical properties of the CCs. The wide application of CCs magnets is, however, impeded by the fact that they cannot work in persistent mode themselves. This is because, due flux creep in high-Tc superconductors and joint resistance, current in the closed superconducting circuit suffers an inevitable decay. One way to solve the problem is to use an external power supply to operate the magnet, but thick current leads have to be used for transporting high current, which induces considerable heat loss. An alternative solution is to use a flux pump. Flux pumps are devices which can inject a direct current into a superconducting circuit without electrical contacts. These can help a superconducting magnet operate in real persistent current mode. Over decades, several flux pumps for low-Tc superconductors have been achieved. The key point for these flux pumps is to drive part of the superconductor normal, either by heat or high magnetic field. An example of such a flux pump is described in H. L. Laquer, K. J. Carroll, and E. F. Hammel, Phys Lett. 21, 397(1966).
For high temperature (Tc) superconductors such as YBCO (yttrium barium copper oxide), it is difficult to use high field to drive it normal, due to its very high upper critical field. Using heat to drive a superconductor normal is also challenging for practical use, because of the low response speed as well as the heat loss. Recently, several HTS flux pumps based on a travelling magnetic wave have been successfully developed.
Classical flux pumps typically use two switches in which the switches are fully opened. That is the switches are sent normal in sequence as flux is drawn first into one loop and then the second destination loop. A general review of the prior art can be found in, e.g. L. J. M. van de Klundert and H. H. J ten Kate, Cryogenics 21, 195 (1981), and L. J. M. van de Klundert and H. H. J ten Kate, Cryogenics 21, 267 (1981).
However, as can be appreciated, there is a need for further improvement of superconducting flux pumps.